The current invention discloses a method of preparing a fabric electrode catalyst with diameters of the fibers having nanometer dimensions. Such nanofiber catalysts can be used in proton exchange membrane fuel cell (PEMFC) and Li-air battery (LAB) applications.
A proton exchange membrane cell (“PEMFC”) is an effective device for energy conversion applications. A PEMFC can convert chemical energy to electric energy through the electro-catalytic reactions. The PEMFC operates at a relatively low temperature with the gas phase hydrogen used as fuel and oxygen (air) used as an oxidant. Due to its high conversion efficiency, low noise and low emissions, a PEMFC is deemed to have high potential in the areas of automobile and distributed power generation.
At the core of a PEMFC is the membrane electrode assembly (“MEA”) which includes an anode, a cathode and a polymer electrolyte layer disposed therebetween. At the surface of the anode, hydrogen is oxidized to protons through an electro-catalytic process,H2→2H+→2e′  (1)The protons thus produced are transported to the cathode side of the cell through a proton conductive membrane. At the surface of the cathode, oxygen is electro-catalytically reduced and subsequently reacts with protons in accordance with equation (1) to form water,O2+4e−−4H+→2H2O  (2)Equation (2) is also known as the oxygen reduction reaction (“ORR”). The reactions of Equations (1) and (2) occur on the surface of electrode catalysts. At present, the most effective catalyst for these reactions are made of platinum supported on amorphous carbon. A typical Pt loading on MEA surface ranges from 0.2 mg/cm2 to 0.4 mg/cm2. Since platinum is a precious metal with very limited supply, its usage adds a significant cost to a PEMFC system. Other platinum group metals (“PGMs”), such as Pd, Rh, Ru, are also being evaluated as a replacement for Pt. They too, suffer from the same issues as high cost and limited reserves. There is thus a strong need to find low cost materials as non-PGM catalysts to replace the usage of PGM materials to lower the overall cost of fuel cell systems.
A rechargeable Li-air battery represents another important electrochemical device that has high energy storage density and potentially high conversion efficiency. A LAB can be generally divided into three key components; an anode and a cathode separated by an electrolyte layer or membrane. The anode is made of lithium metal which exchanges between the ionic and metallic states during the discharge/charge processes. The electrochemical process occurring at the anode surface can be described simply by the following reversible equation:Li→Li++e  (3)The Li+ ion thus formed will be shuttled back and forth between and the anode and cathode through a lithium ion conducting electrolyte membrane during the discharging-charging cycle. For an aprotic LAB, the oxygen is electro-catalytically reduced to oxide ions during the discharging cycle and re-oxidized back to gaseous O2 during the charging cycle (oxygen evolution reaction, or “OER”) at the cathode catalyst surface through a reversible interaction with Li+ ion according to the following reaction:O2+2e−+2Li+Li2O2  (4)The equation (4) is generally called as the oxygen reduction reaction (ORR) for the forward reactions and oxygen evolution reaction (OER) for the reverse reactions, respectively.
At present, there exist a number of technical challenges facing LAB development. The first is the electric energy efficiency for the discharge-charge cycle. The discharging voltage of a LAB is directly affected by the kinetic barrier therefore the overpotential of the forward reaction in (4). Similarly, the barrier of the reverse reaction of Equation (4) influences the charging overpotential, and therefore the voltage. An effective catalyst in the LAB cathode can decrease both discharging and charging overpotentials, thereby improving the electric conversion efficiency. The current cathode catalysts for LAB are typically made of metal oxides supported on high surface area amorphous carbons. Such carbons can often be electrochemically oxidized under cathode environment. Furthermore, porous amorphous carbons often limit the interaction between oxygen and the electrolyte with the catalyst due to lack of sufficient triple-phase boundary and poor mass transfer. Consequently, there is a substantial need for an improved catalyst to remedy these problems.
In typical PEMFC applications, a cathodic oxygen reduction reaction, such as that described by Equation 2 provide hereinbefore, typically occurs at the catalyst surface of platinum supported by amorphous carbons, such as Pt/C. Few catalyst metals were found to have a comparable catalytic efficiency to that of platinum for the ORR. Those found with similar catalytic activity usually are in the precious group metals (“PGM”), such as Pd, Rh, Ir, Ru, in addition to Pt. The PGMs generally carry a high price due to limited reserves worldwide. The use of PGMs for an electrochemical device, such as a fuel cell, add significant cost to the system which therefore creates major barriers for commercialization. It is thus highly desirable to find low cost alternatives to PGMs as the electrode catalyst for fuel cell and similar electrocatalytic applications.
There have been many attempts to identify the replacements for PGMs, mainly through materials involving the transition metal compounds. For example, it has been known that the molecules containing a macrocyclic structure with an iron or cobalt ion coordinated by nitrogen from the four surrounding pyrrolic rings have the catalytic activity to capture and to reduce molecular oxygen. It has been demonstrated in the prior art that ORR catalytic activity can be further improved for such systems containing coordinated FeN4 and CoN4 macrocycles if they have been heat-treated. Recent prior art experiments have shown a similar method of making amorphous carbon based catalyst with good ORR activity by mixing macromolecules with FeN4 group and carbonaceous material or synthetic carbon support, followed by high temperature treatment in the gas mixture of ammonia, hydrogen and argon. A prior art US patent has discussed a method of preparing non-PGM catalyst by incorporating transition metals to heteroatomic polymers in the polymer/carbon composite, and also this art considered a method to improve the activity of polymer/carbon composite by heat-treating the composite at elevated temperature in an inert atmosphere of nitrogen. Other prior art has reported another route of making porous non-PGM electrode catalyst using metal-organic framework material as precursors. Such an approach was later expanded by using an organometallic complex impregnated MOF system. The electrode catalysts prepared through these prior methods are generally in the form of powdered materials. To compensate for the relativity low catalytic activity on single catalytic site in comparison with that of precious metals, more catalyst materials are generally required to prepare fuel cell membrane electrode of the same geometric area. More materials often result in a thicker cathode and hence poorer mass transfer, which is undesirable in PEMFC cathode applications where maximum exposure of oxygen and effectively removing water are critical to the cell performance. To circumvent such issues, nanostructured materials, such as functionalized carbon nanotubes, have been evaluated as non-PGM catalyst to improve the mass/electron transports. For example, it has been demonstrated that Fe/N decorated aligned carbon nanotube could serve as the electrode catalyst for PEMFC. To decorate carbon nanotube (CNT) with a non-PGM catalytic site either through direct synthesis or via post-addition has some significant limitations. For example, a chemical vapor deposition (CVD) step is typically used for direct synthesis of CNT. The CVD mixture must to be vaporized first before decomposition during the CNT formation. Such a requirement limits the types of precursors that can be used to functionalize CNT. Furthermore, forming active site through nanotube growth is not an effective method of integrating high concentration atomic nitrogen into the graphitic layer as part of the catalytic center therefore cannot build highly active catalyst. Adding N-containing transition metal organometallic compound on the preformed CNT surface is another approach to fabricate the catalyst with nano-tubular structure. Such a compound can only be applied on the outer surface of the CNT which again limits the density of the catalytic active site. Furthermore, only the organometallic compounds soluble to the solvent compatible to the CNT can be used in such approach, which again greatly limits the choice of the precursors in improving the catalytic performance.
For the LAB application, ORR and OER occur over the cathode surface during discharging and charging step, respectively. At present, various catalysts including transition metal oxide and precious metals have been used and supported by the carbon materials. Such carbons can be amorphous or graphitic, but in general are randomly agglomerated without ordered nano-architecture. Since the cathode reactions in LAB occur between the interfaces of liquid electrolyte, solid catalyst and gaseous oxygen, maximal mass transfer and interaction are difficult to establish through such a random arrangement, which is similar to the cathodic process in PEMFC. In addition, solid precipitates such as lithium oxides are expected to form and decompose during discharge/charge cycle. Such precipitates can be deposited over the exterior surface of the carbon support, blocking the pores and thereby the access of electrolyte and oxygen into the catalysts inside of the pores. Consequently, many problems remain to be solved.